The invention relates to a method of guiding a multitrack vehicle on a curved path which is defined by the vehicle driver by way of a set steering angle or the like, a curved-path signal representing this desired curved path being guided in a form appropriately revised by an electronic control unit to an actuator influencing the yawing motion of the vehicle. Specifically, the invention relates to a method of guiding a multitrack vehicle on a curved path which is defined by the vehicle driver by way of a set steering angle or the like, a curved-path signal representing this desired curved path being guided in a form appropriately revised by an electronic control unit to a steering actuator influencing the steering angle of at least one steerable vehicle wheel. With respect to the technical background, the interested reader can refer to German Patent Document DE 198 12 238 A1 and German Patent Document DE 197 51 125 A1.
So-called active steering systems for motor vehicles are being developed or are partially already in production which are capable of actively representing, in addition to the steering angle set by the driver by way of the steering wheel (or generally, a so-called steering handle), an additional steering angle at the steerable vehicle wheels (these are normally the front wheels), whereby an additional side force on the vehicle front axle and thus a desired yawing moment can be generated. Such systems are therefore capable of considerably influencing the driving dynamics of a motor vehicle. Such active steering systems may, for example, be a so-called steer-by-wire system or a superimposed steering, which is also known to a person skilled in the art, and by means of which an additional steering angle is mechanically added to the so-called set steering angle of the driver, so that a resulting cumulative steering angle is set at the steerable vehicle wheel.
By means of a so-called active steering system, it becomes possible to implement measures by means of which noticeable improvements of the vehicle driving behavior can be achieved on the basis of the possibility of active steering interventions. In this case, measures or methods are particularly desirable by means of which the dynamic steering response behavior can be improved in that a basically existing phase delay in the transmission between the driver's steering input (for example, at the steering wheel) and the resulting yawing reaction of the vehicle can be reduced.
One possibility in this respect is represented by the principle of the so-called derivative action steering, which is mentioned, for example, in the aforementioned German Patent Document DE 197 51 125 A1, and which is a controlled steering intervention in which a generated additional steering angle is proportional to the steering wheel rotational speed, thus to the time-differentiated set steering angle of the driver. Specifically, the measured set steering angle can be filtered by a simple PD derivative action member and, according to a given steering ratio, can be additionally set at the steerable wheels. The derivative action with respect to the steering angle by the PD derivative action member causes a smaller phase delay in the transmission between the steering input at the steering wheel and the yawing reaction of the vehicle and in this manner ensures a faster response of the vehicle movement to a steering input by the driver.
Another possible method of improving the steering response behavior on motor vehicles consists of a so-called model-based pilot control of the yaw rate. In this case, a so-called reference yaw rate is first determined from the set steering angle, from the vehicle driving speed and from a vehicle reference model and, by means of an inverse vehicle model, a corresponding steering angle for the steerable wheels is determined therefrom and is then appropriately set at these. In the case of a good match between the vehicle model used as the basis and the real vehicle, the thus calculated steering angle provides the desired yawing motion of the vehicle; that is, the actual yaw rate corresponds to the so-called reference yaw rate. By the corresponding selection of the reference model, in this manner, the transmission between the steering input at the steering wheel and the yawing reaction of the vehicle can be adjusted in a targeted fashion, and particularly an improvement of the steering response behavior can be achieved.
The two known methods described above form a so-called pilot control and, in addition to the set steering angle, (i.e., the steering wheel angle defined by the driver), and the actual vehicle driving speed, require no additional information on the momentary vehicle movement. To this extent, a considerable difference exists between such a pilot control and a known control method, as it is described, for example, in the aforementioned German Patent Document DE 198 12 238 A1. In the case of this control method or similar known control methods, a control deviation is derived from the determinable difference between the actual value of the yaw rate and an appropriately determined desired yaw rate value. This control deviation can then be fed for the purpose of a minimization to, for example, two control units which operate independently of one another, i.e., a steering control unit and a braking control unit. Thus, in contrast to a pilot control, in the case of the last-mentioned control method, the actual yaw rate of the vehicle is measured and is adapted to the desired yaw rate, in which case, this desired yaw rate can be maintained by interventions into the vehicle steering system as well as by interventions into the vehicle braking system. Therefore, by an additional steering or braking, a reaction can take place to a deviation between the actually travelled curved path and the desired curved path, which deviation results from some type of influence.
The present invention concerns no control method by which a variance comparison (for example, with respect to the yaw rate) is carried out, but—similarly to the above-mentioned derivative-action steering—it relates to a method for the pilot control only from the two basic input quantities, specifically the so-called “set steering angle” and the “vehicle speed”. Exemplary embodiments of the present invention provide a method with clearly larger freedoms than in the case of the known derivative-action steering. For example, in the case of the known derivative-action steering, system limits exist which are defined by the steering actuating system, for example, with respect to the bandwidth or the regulating speed.
The solution to this task is characterized in that the curved-path signal formed of the set steering angle and of the vehicle speed and, in particular, representing the yaw rate, can be used in the sense of a pilot control for the appropriate change of the longitudinal force at the wheels of at least one vehicle side, so that, in addition to or instead of the setting of the steering angle, in addition or by itself, a longitudinal force can be applied to at least one vehicle wheel in order to travel the desired curved path.
When—as initially indicated—the curved-path signal representing the desired curved path is guided in a form appropriately revised by an electronic control unit to a steering actuator influencing the steering angle of at least one steerable vehicle wheel, the curved-path signal formed of the set steering angle and of the vehicle speed and representing particularly the yaw rate can be used in the sense of a pilot control not only for the appropriate controlling of the steering actuator but also for the appropriate changing of the longitudinal force at the wheels of at least one vehicle side, so that, in addition to, or instead of, the setting of a steering angle, in addition or by itself, a longitudinal force can be applied to at least one vehicle wheel in order to travel the desired curved path.
In accordance with one aspect of the present invention, the above-mentioned longitudinal force is generated by applying a braking force to the wheel or wheels of one vehicle side. However, it is also conceivable to let a driving force act on one side, or to combine the two above-mentioned longitudinal forces with one another such that the wheels are braked on one vehicle side and are accelerated on the other vehicle side. Therefore, for reasons of simplicity, in the following, the application of a longitudinal force is equated with the application of a braking force. This means that, even if in the following only a braking force is mentioned with respect to a vehicle wheel, instead of this braking force, an appropriately large driving force can be applied to a vehicle wheel on the opposite vehicle side, in order to achieve a resulting comparable yawing moment.
According to the invention, a pilot control method is disclosed which permits an improvement of the vehicle handling, or of a yawing motion of the vehicle, by means of integrated or generally combined steering interventions and braking interventions. As result of the targeted braking of individual wheels, an additional yawing motion can be applied to the vehicle which boosts a steering operation. In this manner, even when system limits, for example, of the steering actuating system have been reached, additional improvements of the steering response behavior can be achieved. A method is therefore disclosed for improving the steering behavior, optionally by means of a steering intervention or by means of a braking intervention, or by means of a combined steering intervention and braking intervention, which is a pure yaw rate pilot control requiring only the steering wheel angle (or the like) and the driving speed of the vehicle as input signals. In this case, an arbitrary distribution of the regulating interventions into the steering system and into the braking system of the vehicle can take place. Here, it may also be provided that only the vehicle braking system is appropriately controlled, whereby a steering of the vehicle with a pure braking intervention at individual wheels is permitted, without any locking of the steerable vehicle wheels.
Basically, a first yawing moment fraction can be achieved by the steering angle of the steerable vehicle wheels, and a second yawing moment fraction can be produced by the application of a longitudinal force in the form of a braking force (and/or of a driving force). These two yawing moment fractions cumulatively then result in the yawing moment which is required for the successful and agile traveling of the desired curved path, at least when no significant disturbing influences are present. This last-mentioned yawing moment formed of the sum of the two yawing moment fractions, in the following, will be called an actuator yawing moment, which is to be appropriately applied, and, for the purpose of a pilot control, in the absence of disturbing influences, can be determined solely from the set steering angle and the actual vehicle driving speed.
In this case, the determination of the so-called actuator yawing moment to be applied for the successful traveling of the desired curved path can take place, for example or preferably, by means of a linear single-track model, in which case the actual lateral force of the steerable wheel/wheels resulting from the set steering angle should be returned in order to be able to also take into account the so-called reaction moment of the vehicle. As known, the above-mentioned actuator yawing moment is obtained from the desired yaw rate which—as mentioned above—is determined only from the set steering angle and the vehicle driving speed while taking into account the yaw inertia of the vehicle and its reaction moment, which actuator yawing moment now has to be applied to the vehicle by means of one or several actuators, so that this vehicle moves at the desired yaw rate.
As mentioned above, the division of the actuator yawing moment then taking place according to the invention into a yawing moment fraction, which can be produced by setting a steering angle, and into a yawing moment fraction which can be produced by applying a longitudinal force, can be freely selectable, in which case one of the fractions can also assume the “zero” value. This above-mentioned distribution to a steering actuator and a braking actuator respectively can also take place in a variable manner as a function of marginal conditions. In particular, in this case, the operating speed of the driver's steering handle, that is, the rotational speed of the steering wheel, can be taken into account. In addition, a characteristic-diagram-controlled distribution, for example, by using the vehicle driving speed as a basis, is conceivable. Moreover, a setting limitation of a used actuator, particularly of the steering actuator, can also be taken into account as a marginal condition or influencing variable, as well as the dynamic behavior of the actuator or of the respective concerned actuators. Furthermore, in the event of a system failure, for example, of the steering actuator, the vehicle can still be steered, specifically solely by means of braking interventions.
In a preferred embodiment of the invention, the yawing movement fraction produced by the setting of a steering angle is obtained from a low-pass filtering of the actuator yawing moment to be totally applied for the traveling of the desired curved path. Such a low-pass filtering results in particularly agile dynamics of the vehicle movement because then the “fast” fractions from the desired yaw rate can be converted by almost delay-free braking interventions, while the “slower” fractions are implemented by way of the steering angle of the steerable vehicle wheels. This has the advantage that the respective braking interventions also only have a very short duration and thus lead to no significant deceleration of the vehicle. However, the response behavior upon a steering intention by the driver will then be very fast—as desired.
It is explicitly pointed out that, instead of the yaw rate, also the so-called sideslip angle or the lateral acceleration of the vehicle can be used as the so-called curved-path signal representing the desired curved path. As known, during a progressive vehicle movement without any significant disturbing influences, the relationship ay=v·({dot over (β)}+{dot over (ψ)}) applies, if ay is the lateral acceleration; v is the driving speed of the vehicle; β is the sideslip angle; and {dot over (ψ)} is the yaw rate.
Returning to the division of the actuator yawing moment into a first yawing moment fraction which can be produced by a steering angle and into a second yawing moment fraction which can be produced by the application of a braking force or a longitudinal force, a curved-path signal can now be defined or will be defined, in addition to the yaw rate signal ({dot over (ψ)}), which curved-path signal represents the sideslip angle β or the lateral acceleration ay of the vehicle, the corresponding values being appropriately defined. Thus, a resulting lateral force is then defined, so that the steering locking angle is thereby fixed.
It should also explicitly be pointed out that a steering locking angle at the steerable vehicle wheels can be set not only by means of an actuator but, as in the case of conventional steering systems, also solely virtually manually by the driver of the vehicle. Thus, in addition to the set steering angle and the vehicle speed, from which the so-called curved-path signal is obtained preferably in the form of a yaw rate, the steering angle of the steerable vehicle wheels (or of the steerable vehicle wheel) is also defined. Here, the method according to the invention can also be used because, parallel to the manual steering angle mechanically transmitted or generated corresponding to the steering angle set by the driver, another yawing moment can be produced by way of a longitudinal force applied at least to one vehicle wheel, in which case this additional yawing moment is derived for the purpose of a pilot control solely from the set steering angle and the vehicle speed.